1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to changeover control of a driving power supply in the liquid crystal display device.
2. Description of the Related Art
In recent years, liquid crystal display devices are in wide use in devices for mobile use such as portable devices. In accordance therewith, a demand for low power consumption liquid crystal display devices suitable for outdoor use is increasing and reflective liquid crystal display devices are drawing attention.
The reflective liquid crystal display device is suitable for reducing power consumption and for outdoor use since it constantly utilizes external light as a light source without using any backlight. However, since the reflective liquid crystal display device uses external light as a light source, especially in a liquid crystal display device of an active matrix type, an afterimage remains visible by a user due to residual charges in liquid crystal whenever the power supply is cut off (power-off), which results in deteriorated display quality.
A method of quickly clearing the display at the power-off time in the conventional reflective liquid display device is disclosed in the patent document 1 (Japanese Patent Application Laid-open No. Hei 1-170986). A reflective liquid crystal display device disclosed in the patent document 1 is configured such that a driving power supply is directly fed from a power source of the display device to a source driving circuit for driving source lines (source signal lines) of a display part, and a driving power supply is fed to a gate driving circuit for driving gate lines (gate signal lines) from the power source via a power holding circuit having a capacity large enough to hold a power for a prescribed time.
Then, a power-off signal generated based on the detection of a power-off state is inputted to a gate driving circuit, so that the gate lines connected to the gate driving circuit are all activated (activated to turn on transistors connected to the gate lines) simultaneously using the power held in the power holding circuit. Consequently, residual charges in liquid crystal at the power-off time are discharged in a short time to clear the display, so that an afterimage is prevented from being visible.
Further, another example of the conventional art is disclosed in, for example, the patent document 2 (Japanese Patent Application Laid-open No. 2001-195025).
However, if a driving waveform of gate lines at a normal driving time is a rectangular wave as in the conventional reflective liquid crystal display device described above, unevenness in luminance inclination may possibly occur in a display part between pixels closer to the gate driving circuit and pixels distant therefrom. Methods of solving this unevenness in luminance inclination include a method in which a voltage of a driving power supply (hereinafter, referred to as a “gate-on power supply”) Vgon for activating gate lines is varied with time (in a pulsed manner) to be fed to the gate driving circuit, thereby blunting gate driving waveforms VOUT1, VOUT2, . . . as shown in FIG. 5 (see, for example, the patent document 3 (Japanese Patent Application Laid-open No. 2001-125069)).
The use of such gate driving waveforms VOUT1, VOUT2 . . . whose falling edges are blunted can reduce luminance unevenness in the direction in which the gate lines extend. Note that Voff is a driving power supply for inactivating the gate lines (hereinafter, referred to as a “gate-off power supply”) in FIG. 5. Further, a circuit for generating the gate-on power supply Vgon as shown in FIG. 5 will be referred to as a luminance inclination circuit.
However, the method of reducing luminance inclination unevenness using the luminance inclination circuit has a problem that the power holding circuit that causes residual charges in liquid crystal to be quickly discharged to clear the display at the power-off time as described above is not applicable to the gate driving circuit. In other words, though display quality at a normal driving time is improved, display quality deterioration due to the afterimage at the power-off time cannot be avoided. This is because in the method of reducing luminance inclination unevenness, the voltage of the gate-on power supply Vgon is varied with time at the normal driving time, and if the power holding circuit is used, voltage fluctuation is made small due to its large capacity or the like, which makes it difficult to cause voltage variation (makes it difficult to generate a blunted waveform).